Fluid valves of the plug type are well known. The principal problem posed by such valves is the need to maintain a fluid seal between the rotating valve plug and the valve body. The valve plug is rotatable between an open position where fluid can pass freely through the valve to a closed position where the plug functions to block fluid flow. Failure to maintain a liquid seal will cause the fluid, often at high pressure, to leak around the plug to downstream piping. Further, whether the plug is at the open or closed position, the failure to provide for sealing of the plug can result in fluid leaking around the plug and venting through the plug stem to the environment. Such venting can present health, safety and fire hazards.
In view of the above problems, valves have been devised which include hydraulically actuated seals. For example, our co-pending U.S. application filed Sept. 15, 1982, Ser. No. 418,590, now U.S. Pat. No. 4,497,467, and titled "PLUG VALVE WITH HYDRAULICALLY ACTUATED SEALS", the subject matter which is incorporated by reference herein, discloses seals operated hydraulically to seal the plug within the body when the plug is at the closed position.
It has been found that in certain circumstances where the temperature of the fluid or the environment may vary or where the temperature is other than that for which the valve has been designed, expansion of the hydraulic fluid can create problems. Expansion of the fluid in a closed hydraulic system such as described in the patent application identified above, increases the hydraulic system pressure and, thereby, the pressure imposed upon the seals. One problem resulting from the increased pressure is that the valve may "freeze" in the closed position since the increased hydraulic pressure causes the seals to enagage the plug with such force to prevent rotation of the plug toward the open position. Even should the plug be freed for rotation, the increase in pressure may be such that the seals undesirably bind with the rotation of the plug. This tends to wear the seals and/or plug and possibly cause damage thereto. A still further problem is that the increased pressure may cause the hydraulic fluid to leak past sealing rings or the like eventually resulting in loss of hydraulic fluid and pressure.